The trend in the design of power systems for electronic assemblies has been towards distributed power architectures. A power system with a distributed power architecture may employ many small board mounted power supply modules in place of a few larger and more centralized power supply modules. The power system may be used to power a diverse variety of electronic assemblies, including, for example, a computer work station, a file server or a telecommunications switching system. Each board mounted power module may be conveniently located proximate the electronic circuitry being powered. Often, one or more board mounted power supply modules are located on a circuit card in the electronic assembly. Since real estate on the circuit card is limited, minimizing the size of the power supply modules is a continuing goal. By reducing a footprint of the power supply module, real estate on the circuit card previously required by the larger power supply modules may be used for additional circuitry, for instance, to increase the processing throughput of a computer card or to increase the switching capacity of a telecommunications card. Additionally, many electronic assemblies employ a circuit card to circuit card spacing of less than one inch. Minimizing the circuit card to circuit card spacing allows for a denser assembly, which allows the throughput or capacity of the electronic assembly containing the circuit cards to be advantageously increased. A power module having an attribute of a low height profile is also more desirable.
Consequently, the trend in the design of power supply modules has been toward achieving increased output power along with a lower height profile and a smaller footprint area, thereby increasing power density. Improvements in power level, power density or profile, however, cannot be made at the expense of the thermal and electrical characteristics of the overall power supply module and its constituent components.
Conventional power supply modules may be constructed as a unitary, encapsulated package, having one or more rows of leads, with the power supply module enclosed in a metal case. The leads allow the module to be coupled to a circuit card while the metal case contains attachment mounts for an external heat sink. The power supply module often includes one or more power devices (e.g., transistors or diodes) in thermal communication with the metal case, one or more magnetic devices (e.g., transformers or inductors) providing electrical isolation and energy storage and one or more circuit boards containing passive electronic devices to provide, among other things, control and monitoring functions.
Power devices and magnetic devices that require thermal management due to their high power dissipation may be mounted on a metal circuit board employing insulated metal substrate technology, for example, a THERMAL CLAD substrate manufactured by the Bergquist Corporation of Minneapolis, Minn. Electronic devices such as passive devices that do not require thermal management may be mounted on either the metal circuit board or on a conventional FR4 circuit board. The FR4 circuit board may then be mechanically and electrically coupled to the metal circuit board to facilitate electrical communication and power flow between the various parts of the power supply module.
The leads of the power supply module are mechanically and electrically coupled to either the FR4 circuit board or the metal circuit board. The power supply module is typically encapsulated in a plastic or metal case that is filled with an encapsulant to protect the internal components of the power supply module from contaminants and perhaps to improve heat flow between the internal components and the case.
The aforementioned encapsulated package design, however, suffers from a number of deficiencies. At least two circuit boards, the metal circuit board and the FR4 circuit board, are required to accommodate the constituent components of the power supply module. Employing multiple circuit boards increases both complexity and cost of the power supply module. Further, the encapsulated package design is not readily mass producible, for example, with conventional pick and place equipment.
A power supply module employing the encapsulated package design is often coupled to a heat sink that dissipates some of the heat generated by the power and magnetic devices. There are applications, however, where the heat sink is not required due to the power supply module's lower power dissipation. Other applications may require a power supply module having a lower height profile than is available with conventional encapsulated package power supply modules. An open frame design is typically employed in these applications. An example of an open frame power supply module is the HW100 series manufactured by Lucent Technologies of Mesquite, Tex. (Lucent). An open frame power supply module typically includes a number of electronic devices mounted on a single FR4 circuit board. The leads of the power supply module are mechanically and electrically coupled to the FR4 circuit board to allow the power supply module to mount to the end user's circuit card.
Conventional encapsulated and open frame power supply modules are often mounted to the end user's circuit card via through-hole pins. The leads of the power supply modules are typically soldered to the circuit card manually. Lucent's JW150 series or HW100 series board mounted power supplies (BMP's) are examples of through-hole mounted power supply modules. An end user's circuit card typically contains a large number of surface mount components. In fact, the power supply module is often the only through-hole mounted component on the circuit card. A separate or additional manufacturing step is thus required to mount the power supply module to the circuit card, thereby increasing the complexity and overall cost of the electronic assembly incorporating the circuit card. Therefore, it would be advantageous to provide a power supply module capable of being surface mounted to the circuit card using the same reflow soldering process used to mount the other components.
One difficulty with the surface mount approach lies in the need for the power supply module to pass through the reflow soldering process as it is mounted to the end user's circuit card. The reflow soldering process can subject the power supply module to extreme stresses, possibly melting all of the power supply module's internal solder joints and possibly degrading the functionality of the constituent components of the power supply module [e.g., the equivalent series resistance (ESR) of tantalum capacitors]. The electronic devices of the power supply module may shift or even decouple from the FR4 circuit board during the reflow soldering process, possibly destroying the functionality of the power supply module.
Efforts to alleviate the stresses caused by the reflow soldering process have often centered on the use of high temperature solder for the power supply module's internal solder joints. The end user's reflow temperature profile may be set to a temperature that is sufficient to melt the solder between the power supply module and the circuit card, but is too low to melt the high temperature internal solder joints. This may require special processing techniques and materials, such as employing lead-free solder and lead-free component plating to ensure high reliability. The '753 application describes a lead-free solder process that improves the reliability of solder joints that may be subjected to the reflow soldering process. Also, it may be necessary to secure heavier components using glue or other mechanical fasteners to ensure that the components remain in place during the reflow soldering process. Clearly, employing glue or additional mechanical fasteners will add cost and complexity to the overall assembly.
Accordingly, what is needed in the art is a surface mountable power supply that overcomes the deficiencies of the prior art.